Interference can reduce the reliability of wireless (radio) communications. When an interfering source is near a receiver, and when the interference source generates signals with sufficient power in a frequency band of interest in a wireless channel between the source and the receiver, the power of a desired signal at the receiver can be much lower than the power of the interference.
A blanking technique is generally used to remove signal samples that are subject to the interference. For example, if the interfering source generates a wide-band signal, some time samples in the received signal can be corrupted by the interference. The receiver can replace these time samples with a high power by zeros, thus blanking out both the desired signal and interference in these time samples. Then, the desired signal can later be decoded if a correcting procedure is used.
Narrowband Symbols and Tones
As defined herein, an OFDM signal includes data symbols and pilot symbols. Because OFDM symbols are transmitted as frequency tones in subcarriers of the wireless signal, the data symbols and pilot symbols are often referred to as data tones and pilot tones. Hereinafter, the terms symbols and tones are used interchangeably.
If the interfering source generates the narrowband signal, then the receiver can replace corrupted interfering frequency tones by zeros, thus blanking out both the desired signal and the interference at these specific frequency tones.
For narrowband interference, the effectiveness of blanking techniques depends on the ability of the receiver to determine the frequency tone at which the interference is present. If the coherence bandwidth of the wireless channel is larger than the bandwidth of a wireless transmission, then the interference can be detected by comparing the received power of each received frequency tone. This is relatively simple.
However, when the transmission requires a bandwidth that is larger than the coherence bandwidth of the wireless channel, then the receiver needs to estimate both the channel characteristics as well as possible interference at the same time. This is difficult.
The IEEE 802.22-05/0262r0 standard “Orthogonal Interference Detection” specifies how to concurrently estimate channel characteristics and the interference. Within a coherent frequency band, at least two pilot tones are transmitted by a transmitter. By definition of coherence bandwidth, these two pilot tones have similar received power. When the difference between the received powers of the two pilot tones is large, the presence of interference is likely. It is explicitly stated in the IEEE 802.22-05/0262r0 standard that “two symbols in one group must be in the same coherence bandwidth, but the symbols in different groups do not need to satisfy [the interference detection formula]”. In other words, the prior art does not use pilot tones from outside of the coherence band to determine interference within a band.
It is desired to provide a method and system for joint wireless channel estimation and narrowband interference.